Pause the game and press R1, Circle, R1, L2, R1, L1, Square.
In the military base level where you have to rescue general Carington, there is a door on the left that leads to some kind of bedroom just before the room with the shaft leading to Carington's prison cell. Enter the room and take out the three SPADS soldiers in the room. At the opposite end of the room to the right is a locker standing by itself. Open it to get a 9 mm pistol, and the Dual Weapons skill will be enabled. From now on you will be able to use dual pistols and miniguns.
At the start of the game, run though the back door to a gun faster and safer than going to the man at the front. Note: Hit him in the head and he will die faster.
When in sniper mode, zoom in. To steady yourself for greater accuracy, sit there and do not touch anything to look or move for about five secounds or until he stops moving around. You can now look around with perfect dead aim accuracy.
Wait until an enemy reloads his weapon before killing him. Kill him before he fires his gun and you will get a full magazine for your weapon.
When you first start the FBI level, immediately after the woman shoots out your cuffs, go to the center of the room with the three computers. You will find the Ubisoft logo acting as a screensaver. Also, if you walk up to the computer screen, look down at it, and press X, the screen will say "GAME OVER".